This application claims the priority of German patent document 198 27 880.2, filed Jun. 23, 1998, and PCT International Application No. PCT/EP99/03168, filed May 08, 1999, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a circuit configuration for a fuel cell system comprising a plurality of individual fuel cells.
In H2/O2 composite fuel cell systems, operating states can arise in which individual cells change their polarity. The magnitude of the cell voltage is about 0.5 V in this case. In this operating state, the cell acts as a load and, at the same time, is heated to an impermissibly high level or is even destroyed. By way of example, electrical energy can be generated in a mobile energy generating system by means of a composite fuel cell system and can in turn be used for driving electric machines.
A composite fuel cell system comprises a plurality of individual fuel cells connected in series. Each individual fuel cell supplies the rated current IN of the composite fuel cell system and the sum of the voltages of the individual fuel cells produces the rated voltage UN of the composite fuel cell system. The generation of electrical energy takes place in the individual fuel cells by means of an electrochemical reaction of, for example, hydrogen, hydrogen-containing gas, methanol or the like. During operation, the hydrogen and/or oxygen supply may fail within individual fuel cells. As a result, depending on the substance that is missing, the voltage potentials of the anode or of the cathode are shifted, and the voltage of this individual fuel cell drifts from the positive to the negative range. This means that this fuel cell undergoes a polarity reversal; and because it this fuel cell is connected in series with the remaining fuel cells which have not undergone polarity reversal, the load current generated by the latter is impressed onto the fuel cell which has undergone polarity reversal. The fuel cell which has undergone polarity reversal therefore becomes the load and is heated on account of the power which it dissipates, so that it may be destroyed. Depending on the evolution of heat, under certain circumstances the entire composite fuel cell system may also be destroyed.
In order to avoid this result, a known procedure is to detect the voltages of the individual fuel cells. If it is determined that the voltage of an individual fuel cells is dropping excessively or is already undergoing polarity reversal, the gas feed to the entire composite fuel cell system is stopped in order to avoid destruction of the composite fuel cell system.
Against this background, one object of the present invention is to provide a circuit arrangement, and a method operating it, which can avoid destruction of or damage to the composite fuel cell system or individual cells in the event of a fault in individual fuel cells.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which the voltages of individual fuel cells are monitored, and circuit components are provided in the form of a low-resistance parallel circuit, (for example, diodes) for bridging individual fuel cells. Such bridging is implemented if the magnitude of their voltage falls below a specific threshold value or if the sign of a cells voltage has changed relative to normal operation. Alternatively, controllable switches are provided, which are driven if the composite fuel cell system is intended to be discharged.
Owing to the fact that individual defective fuel cells can be bridged, the composite fuel cell system can remain in operation, supplying the electrical energy that is generated by the remaining fuel cells.
Automatic regulation can be effected by selecting the diodes in such a way that the forward voltage of the diodes has a value at which the individual fuel cell should expediently be bridged owing to polarity reversal. During normal operation of the individual fuel cells, the diodes are reverse-biased.
The use of controllable switches, as mentioned above, allows the bridging of individual fuel cells to be performed more flexibly. By way of example, these controllable switches may already be driven when the voltage of the individual fuel cells has not yet undergone polarity reversal but has fallen below a specific threshold value, which may be 0.5 V for example. Moreover, when using controllable switches, it is possible to discharge the composite fuel cell system during maintenance work by driving the controllable switches, thereby avoiding danger to persons during the maintenance work that is to be carried out.
In one embodiment of the circuit arrangement according to the invention, the controllable switches are MOS field-effect transistors, which can be driven with minimal power losses, even when the switched powers are comparatively large.
In another embodiment of the circuit arrangement according to the invention, the components are fitted externally to the composite fuel cell system, which is advantageous if the contact-connection of the components has a large area, in order to minimize the current density (and thus the local evolution of heat). Furthermore, it is advantageous if the heat can be dissipated in a simple manner. The contact-connection of the components can be effected between the graphite plates.
In still another embodiment of the invention, the components are integrated in the composite fuel cell system. This simplifies the production process because the components do not have to be present as separate structural parts. Furthermore, it is possible in this case to limit the current density by correspondingly suitable dimensioning of the structural parts. In an advantageous refinement, in accordance with this embodiment, the components are integrated in the edge of the composite fuel cell system, simplifying the contact-connection of the components, because the latter touch the graphite plates directly. Moreover, good heat dissipation is ensured on account of the components being introduced into the edge.
In yet another embodiment of the invention, the components are distributed over the cross-sectional area of the composite fuel cell system. As a result, the components no longer form additionally structural parts, thereby simplifying the production sequence.
In a further embodiment, a cooling system is integrated in the composite fuel cell system, so that a heat loss which arises in the components (and cannot be output directly to the surroundings) is advantageously dissipated.
In another embodiment a controllable switch is driven via the output of an amplifier or comparator, which output is connected to the control terminal of the controllable switch, which has an input connected to the terminals of the individual fuel cell. The amplifier or comparator is supplied with energy by connection with the terminals of a battery arrangement via current sources or current sinks. A potential shifting element furthermore is connected in parallel with the terminals for supplying energy, with a voltage dividing element connected in parallel with this potential shifting element. The tap of this voltage dividing element is connected to the other terminal of the individual fuel cell.
The potential shifting element may be a zener diode, for example. The voltage dividing element may be a potentiometer or a subdivided zener diode.
This advantageously makes it possible to supply power from a battery arrangementxe2x80x94for example also from the composite fuel cell system itselfxe2x80x94using an internal reference voltage of the amplifier or comparator. This circuit arrangement avoids the problems which arise as a result of a potential shift depending on which of the individual fuel cells is to be examined. Furthermore, the current sources take up possible operating voltage fluctuations in the battery arrangement.
The controllable switch may be a MOS enhancement-mode or depletion-mode transistor of the n- or p-channel type, or a Darlington bipolar transistor of the npn or pnp type. The amplifier or comparator (inverting/non-inverting) and the expedient assignment of the operating voltage reference point are determined by the choice of the corresponding controllable switch.
In another embodiment, particularly in the case of the fuel cells which are arranged at the periphery of the composite fuel cell system, a potential shifting circuit is arranged between one terminal of each individual fuel cell and the input of the amplifier or comparator, and also between the tap of the voltage dividing element and the other terminal of the individual fuel cell. In this manner, it is advantageously possible to achieve a sufficient voltage level also for monitoring the individual fuel cells situated at the edge of the fuel cell system.
In a further advantageous embodiment of the invention, a potential shifting circuit is provided between the output of the comparator or amplifier and the control terminal of the controllable switch. Moreover, it is also advantageous in accordance with claim 13 to design the circuit arrangement in such a way that the voltages of the individual fuel cells are evaluated by means of an amplifier or comparator whose output is connected to at least one optocoupler, the controllable component being controlled via the optocoupler.
Particularly when monitoring individual fuel cells which are arranged at the extremities of the composite fuel cell system, it is possible to achieve a sufficient voltage for driving the controllable component.
The circuit arrangement according to the invention, may advantageously be designed as an integrated circuit.
In a method for operating the circuit arrangement according to the invention, the fuel cells are bridged if the magnitude of their voltage falls below a specific threshold value or if the sign of their voltage has changed relative to normal operation. This can be effected by suitable setting or dimensioning of the circuit arrangement.
The controllable switches are driven if the composite fuel cell system is intended to be discharged. As a result, it is possible to ensure the safety of persons, for example during maintenance work on the composite fuel cell system, if the composite fuel cell system is discharged before the maintenance work is carried out.